Electronic equipment used in home, office or central office must meet regulatory standards for Electro-Magnetic Compatibility (EMC) in the country where the equipment is sold and used. Exemplary standards include FCC Part 15 in the USA and EN55022 in the EU.
EMC generally pursues two different issues, emission and immunity. Emission issues are related to the unwanted generation of electromagnetic energy by some source, and to the countermeasures which should be taken in order to reduce such generation and to avoid the escape of any remaining energies into the external environment. Immunity or susceptibility issues, in contrast, refer to the correct operation of electrical equipment, often referred to as the victim, in the presence of unplanned electromagnetic disturbances.
Before EMC-approval, different tests targeting both emission and immunity issues are performed. Radiated emission tests measure electromagnetic output from the product, that is, both intentional and undesired electromagnetic radiation. EMC measurement standards commonly set limits on how much power a product may emit over a given set of frequencies. Radiating too much at a given frequency may have adverse effects on nearby electronic equipment or radio transmissions. Immunity tests, on the other hand, ensure that the product will not malfunction when exposed to reasonable amounts of electromagnetic noise or interference (EMI) from nearby equipment.
Any equipment exceeding a limit on radiated emission at a given frequency may not be sold and used where the limit applies. For some types of electronic circuits, in particular when equipment comprises a large number of circuits operating at a same fixed frequency, these limits may be very difficult to meet.
One method of reducing radiated emissions is to enclose the equipment in a grounded metal chassis. However, breaches in the chassis may allow electromagnetic emissions to escape or leak out. This is a particular problem for equipment that has removable and replaceable parts. An example of such an equipment is shown in FIG. 1, which depicts a highly scalable and flexible chassis-based platform for high-definition video conferencing and voice communication 1. Ten different modules 2, or “blades,” may be plugged into the chassis, where the blades may be ISDN gateways, MCUs, Telepresence servers, and supervisor modules. The chassis shown in FIG. 1 further comprises a backplane having 180 connections for high speed communication between the blades, wherein each of the connections operate at 6.4 Gb/s.
Electromagnetic radiation at high frequencies, such as in the GHz range (1 Gbps serial communication yields 1 GHz electromagnetic radiation), has a short wavelength, typically in the cm-range (e.g. 6.4 GHz gives a wavelength of approximately 4.5 cm). Hence, the radiation is able to pass through any small slots or gaps in the chassis. In addition to gaps or small slots, which are almost impossible to avoid in products manufactured from sheet metal, holes in the chassis for cooling, etc. can also make it almost impossible to rely on a grounded metal chassis to avoid excessive electromagnetic radiation when communicating on very high frequencies.
A second method of reducing radiated emission is to slow down the speed of the communication, i.e., to reduce the clocking frequency of the communication. For real-time, processing-intensive applications, such as high definition video conferencing, this is clearly not a viable solution.
A third common method for reducing emission of electromagnetic radiation at a given frequency is spread spectrum clock generation (SSCG). Clock driven systems have a narrow frequency spectrum due to the periodicity of the clock. A perfect clock signal would have all the energy of the clock concentrated at a single frequency and at its harmonics, and would therefore radiate energy with an infinite spectral density. Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread at the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed regulatory limits. SSCG modulates the frequency of the clock within a device when transmitting so that the bandwidth of the emissions is increased and therefore an average, or quasi-peak, receiver measurement centered at a given frequency is reduced, i.e., there is a reduction in spectral density. However, in some systems, altering the reference clock has a detrimental effect on the stability of transceivers in the system. Further, in a system having several blades, as depicted in FIG. 1, additional and complex hardware would be required to synchronize the reference clock modulation between the blades.
Further, SSCG does not reduce the total energy radiated by a system and therefore does not necessarily make the system less likely to cause interference. SSCG merely takes advantage of the EMC testing procedures, wherein the measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into narrow frequency bands. A clock-driven system would typically radiate all of the system energy into one frequency and its harmonics, and thus the measuring receivers would register a large peak at the monitored frequency band, thereby increasing the likelihood for exceeding statutory limits. SSCG, on the other hand, distributes the energy so that the energy falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits.